1. Field of the Invention
The present invention relates to a plate type evaporator.
2. Description of the Prior Art
Generally, in a plate type evaporator, alternate chnnels for a liquid to be evaporated and a heating medium are defined between a plurality of vertically extending plate elements assembled face-to-face, wherein the heating medium is fed to the heating medium channels while the liquid to be evaporated is fed to the liquid channels, so that indirect heat exchange takes place therebetween through the plate elements. As a result of such heat exchange, the heating medium if it is in gaseous state, is condensed, with the latent heat of condensation being used to evaporate the liquid in the adjacent channels through the plate elements. Thus, the heat transfer is effected while the liquid is boiling on the heat transfer surfaces of the plates.
When boiling begins in a saturated liquid, vapor bubbles will be generated with the dirt and air particles contained in said liquid serving as nuclei. In the case of so-called pool boiling in a liquid at rest, bubbles are continuously evolved from particular points on the heat transfer surface until the temperature difference between the heat transfer surface and the saturated liquid reaches a certain value. The points of evolution of bubbles on the heat transfer surface are called the nuclei of boiling and the boiling in the described state is called nuclear boiling. It is known that in nuclear boiling, the bubbles evolved act to stir the heated liquid in the channel to accelerate the boiling heat transfer.
When the heat transfer surfaces of the plate elements are flat, bubbles can hardly be evolved at the lower ends of the liquid channels because of a relatively high pressure due to the potential heads, so that it is impossible to fully achieve the effect of accelerating the generation of vapor which can be brought about by the evolution of bubbles stirring the liquid.
In this type of evaporators, various expedients have been adapted to improve the evaporation heat transfer coefficient in order to increase the efficiency of evaporation. For example, such expedients include a heat transfer surface formed with corrugations, a heat transfer surface provided with a layer of porous material, etc. In the former, the heat transfer surface is formed with vertically extending corrugations to provide therealong thick and thin regions in the flow of a fluid to be heated so that the portion of the liquid in the thick regions where heat is concentrated is caused to positively evaporate, while the portion of the liquid in the thin regions, after being heated, is allowed to flow to be added to the thick regions which are evaporating, to thereby increase the efficiency. The latter expedient is intended to cause the nuclear boiling of the liquid in the pores of the porous layer on the heat transfer surface so as to efficiently evaporate the liquid.
However, each expedient is designed only to provide a region for easy heat transfer and concentrate heat in said region so as to produce vapor concentratedly at said region. In other words, the vapor generated grows to a certain degree and leaves the heat transfer surface by the action of its buoyancy, but since such leaving is effected in a stationary state, the time from the time bubbles are evolved until they leave the heat transfer surface is prolonged. As a result, the bubbles remain between the heat transfer surface and the liquid until they leave the latter, so that they cut off the transfer of heat therebetween, thereby lowering the heat transfer coefficient. This problem becomes more serious particularly in the case of a porous heat transfer surface. That is to say, such porous heat transfer surface is intended to accelerate the evolution of bubbles by causing the nuclear boiling of the liquid in the pores, as described above, but undesirably, the bubbles evolved in the pores collide with the liquid flowing into the spaces vacated by the bubbles when they leave the pores, so that the movement of the bubbles is slowed down. This means that the period of time the bubbles cut off the transfer of heat between the liquid and the heat transfer surface is prolonged, thereby lowering the heat transfer coefficient. As a result, it is impossible to fully exhibit the effect of accelerating the evolution of bubbles which features a porous surface.
As is known in the art, the plate type evaporator comprises a plurality of heat transfer plates combined to define boiling liquid channels and heating medium channels alternately therebetween, the arrangement being such that a heating medium is fed into the heating medium channels, so that the boiling liquid filling the boiling liquid channels is evaporated through the heat transfer surfaces by the sensible heat or latent heat of condensation of the heating medium, the heat transfer being effected while the liquid in the boiling liquid channels is boiling on the heat transfer surfaces. That is, with the single-bubble producing mechanism, as is known in the art, tiny bubbles are produced in the cavities on the wall surfaces and gradually grow until they separate from the wall surfaces, this action being repeated to produce vapor. The greater the frequency of the production, growth and separation of bubles, the greater the evaporation capability. The cvaporation capability is proportional to the difference in temperature between the boiling liquid and the heating wall surface and to the heat transfer coefficient associated with the boiling liquid side, while the heat transfer coefficient is proportional to the degree of bubbling which means the frequency of the occurrence of boiling bubbles and to the difference in temperature between the boiling liquid and the heating wall surface and heavily depends on the shape or nature of the heat transfer surface. Where the same quantity of heat is to be transferred, therefore, the formation of a boiling heat transfer surface which provides a high degree of bubbling will ensure a greater evaporation capability even if the temperature difference is small.
The conventional smooth-surface plate has a low heat transfer coefficient and a low degree of bubbling, so that its evaporation capability is low. In the case of a porous boiling heat transfer surface typified by zeolite, the degree of bubbling is high, and bubbles remaining in the pores prevent entry of the boiling liquid and allow activated cavities to be present at all times to accelerate boiling. However, there has been observed a drawback that once the spaces in the porous material are filled with liquid, activated cavities no longer exist and boiling is not accelerated.